This invention relates in general to inflatable vehicle occupant restraint systems, also known as air bag restraint systems. More particularly, the invention relates to a novel and improved filter construction for an inflator of such a system and an inflator module employing such a filter construction, and a method of making such a filter.
Inflator modules for air bag restraint systems of the pyrotechnic type typically include a housing which contains a pyrotechnic igniting charge and a pyrotechnic gas generating material. Usually, separate sections or chambers are provided respectively for the igniting charge and the gas generating material. The housing defines internal ports for the combustion gases of the igniting charge to contact the gas generating material, causing a reaction to produce a larger quantity of gas. Cooperatively, the housing includes a number of exit ports for allowing the generated gas to pass into and inflate an inflatable cushion.
Commonly, the gas generating material produces a relatively high temperature gas which contains a quantity of particulate material. Accordingly, it is common to filter the gas before it passes through the housing ports and into the inflatable cushion, both to remove the particulate material and to cool the gas somewhat.
For driver side air bags, the housing, as well as the inflatable cushion, are typically located in the steering wheel, and typically, the housing is formed as a short cylinder with its longitudinal axis parallel to the axis of rotation of the steering wheel. Passenger side air bags usually employ a cushion of larger volume, and commonly more space is provided for the mounting of the inflator than in the case of the driver side air bag mounted in the steering wheel. Thus, a passenger side inflator housing is typically formed as a relatively elongated cylinder which may be somewhat smaller in diameter than a driver side inflator but provides additional volume through its increased axial length for accommodating additional gas generating material.
In the case of a driver side inflator, the gas exit ports for delivering gas to the inflatable cushion may be placed either in a radial arrangement or an axial arrangement. In axial designs, the ports are located on the upper longitudinal end of the cylindrical inflator housing, whereas in radial designs, the ports are located on the radially outer face of the cylindrical inflator housing. The present invention relates to the latter or radial design.
In the radial arrangement, the filter is generally a tubular member located radially inwardly of the housing exit ports. In the past, the filter has been constructed as a composite filter structure having a number of layers of filtering or other materials which are assembled to provide the composite filter unit. However, the present invention provides a much simpler filter employing a unitary, integrally formed ceramic filter element of a generally tubular form. The filter element may be formed from, but is not limited to, ceramic, powdered metal, metal fiber, or a combination of two or more of these porous, relatively brittle materials.
In order to cause the flow of gas through as large an area of the filter as possible so as to achieve both efficient filtering and maximum gas flow, it is often desirable to have a plenum space between the filter and the inner wall of the inflator housing. However, in the case of a unitary filter, the outer peripheral wall of the filter will lack external circumferential or radial support, by virtue of being spaced apart from the inner wall of the housing rather than being placed thereagainst. Generally speaking, the unsupported tubular filter element will experience hoop stress upon the rapid generation of gas and resultant gas pressure exerted radially outwardly on the filter. Lack of radial external support can result in tensile failure of the generally tubular or ring-like filter due to this tensile hoop stress.
A second type of air bag system of the so-called hybrid/blowdown design as described in U.S. Pat. No. 5,290,060 employs a somewhat simpler pyrotechnic igniter structure in connection with a sealed tube of gas (usually argon) which is coupled to the inflatable cushion. The igniter member serves to dislodge a seal from the tube of gas and allow the gas to rapidly fill the inflatable cushion. While filters are presently not employed in this latter type of design, a filter in accordance with the invention could be so employed if desired.